The present invention relates to a 2-D image detector capable of detecting an image by using radial rays, such as X-rays, visible light, infrared light, etc.
A conventionally known 2-D image detector for detecting an image by using radial rays is provided with two-dimensionally aligned semiconductor sensors which generate charges (electrons-holes) upon detection of X-rays and electric switches which are individually attached to the sensors, so that the charges are read out from the sensors per column by sequentially turning on the electric switches per row. Detailed arrangements and operation principle of such a radiographic 2-D image detector are described in, for example, xe2x80x9cA New Digital Detector for Projection Radiographyxe2x80x9d SPIE, 2432, pp. 237-249, 1995, D. L. Lee, et al., xe2x80x9cApplication of a-Si Active-Matrix Technology in an X-Ray Detector Panelxe2x80x9d, SID 97 DIGEST, pp 91-94, 1997, L. S. Jeromin, et al., Japanese Laid-open Patent Application No. 342098/1994 (Japanese Official Gazette, Tokukaihei No. 6-342098, published on Dec. 13, 1994), etc.
The following will explain an arrangement and an operation principle of the conventional radiographic 2-D image detector.
FIG. 12 is a view which schematically explains an arrangement of the conventional radiographic 2-D image detector, and FIG. 13 is a cross section which schematically shows an arrangement of each pixel of FIG. 12.
As shown in FIGS. 12 and 13, the conventional radiographic 2-D image detector comprises an active matrix substrate having a glass substrate 51 on which are formed an X by Y matrix (hereinafter, referred to X-Y matrix) of electrode wires (gate electrodes 52 and source electrodes 53), thin film transistors (TFTs) 54, charge accumulating capacitors (Csxe2x80x2) 55, etc. In addition, a photoconductive film 56, a dielectric layer 57, and a top electrode 58 are formed almost entirely on the active matrix substrate.
Each of the charge accumulating capacitors 55 comprises a charge accumulating capacitor electrode (Cs electrode) 59, and a pixel electrode 60 which is connected to the drain electrode of the TFT 54. The two electrodes 59 and 60 are placed to oppose each other through an insulating layer 61.
The photoconductive film 56 is made of a semiconductor material which generates charges (electrons-holes) upon irradiation of radial rays, such as X-rays. The semiconductor material used in the aforementioned publications is amorphous selenium (a-Se) which has good resistance to darkness and shows excellent photoconductivity upon X-ray irradiation. The photoconductive film (a-Se) 56 is formed in a thickness ranging from 300 to 600 xcexcm by means of vacuum deposition.
The active matrix substrate can be the one manufactured in the liquid crystal display manufacturing process. To be more specific, an active matrix substrate used for an active matrix liquid crystal display (AMLCD) is provided with thin film transistors (TFTs) made of amorphous silicon (a-Si) or poly silicon (p-Si), and an X-Y matrix of electrodes and charge accumulating capacitors (Csxe2x80x2). Thus, the active matrix substrate manufactured in the liquid crystal display manufacturing process can be used as an active matrix substrate for the radiographic 2-D image detector by slightly changing the specification.
Next, the following will explain the operation principle of the above-arranged radiographic 2-D image detector.
When radial rays are irradiated on the photoconductive film 56, such as the aforementioned a-Se film, the charges (electrons-holes) are generated therein. As shown in FIGS. 12 and 13, the photoconductive film 56 is electrically connected to the charge accumulating capacitors 55 in series. Thus, if a voltage is applied across the top electrode 58 and charge accumulating capacitor electrodes 59, the charges (electrons-holes) generated in the photoconductive film 56 start to migrate to the positive electrode end and negative electrode end, whereby the charges are accumulated in the charge accumulating capacitors 55.
Here, a charge blocking layer 62 made of a thin insulating layer is formed between the photoconductive film 56 and charge accumulating capacitors 55. The charge blocking layer 62 functions as a blocking photodiode which inhibits charge injection from one of the surfaces thereof.
The charges accumulated in the charge accumulating capacitors 55 in the above-described manner are released to the outside from the source electrodes S1, S2, S3, . . . , Sn by opening the TFTs 54 with input signals to the gate electrodes G1, G2, G3, . . . , Gn. Since all of the electrode wires (gate electrodes 52 and source electrodes 53), TFTs 54, and charge accumulating capacitors 55 are provided in an X-Y matrix arrangement, by linesequentially scanning input signals to the gate electrodes G1, G2, G3, . . . , Gn, 2-D image information of the X-rays can be obtained.
In case that the 2-D image detector employs the photoconductive film 56 which shows the photoconductivity to visible light or infrared light in addition to the radial rays, such as the X-rays, the 2-D image detector can also detect an image by using visible light or infrared light.
As has been mentioned, the conventional radiographic 2-D image detector uses a-Se as the photoconductive film 56, but a-Se has drawbacks that the responsivity is poor due to a unique property to an amorphous material, namely, dispersive photoconductivity for a photoelectric current, and that information can not be read out until the charge accumulating capacitors (Csxe2x80x2) 55 are fully charged by irradiating X-rays for a long time due to its poor sensitivity (S/N ratio) to X-rays.
The dielectric layer 57 is formed between the photoconductive film (a-Se) 56 and top electrode 58 so as to (1) prevent the charge accumulation in the charge accumulating capacitors 55 caused by a leak current during X-ray irradiation, (2) reduce a leak current (dark current), and (3) protect the components from a high voltage. In this case, however, a sequence to remove residual charges in the dielectric layer 57 in every frame is additionally necessary. Thus, the radiographic 2-D image detector has a problem that it can be used only to shoot still-frame images.
In order to obtain image data for motion pictures, instead of a-Se, the photoconductive film 56 must be made of a photoconductive crystalline (or poly-crystalline) material having excellent sensitivity (S/N ratio) to X-rays. By using the photoconductive film 56 with better sensitivity, the charge accumulating capacitors 55 can be fully charged by irradiating X-rays in a short time without applying a high voltage to the photoconductive film 56. Consequently, the dielectric layer 57 can be omitted.
Known examples of the photoconductive material with excellent sensitivity to X-rays are CdTe, CdZnTe, etc. In general, photoelectric absorption of X-rays is directly proportional to the fifth power of the effective atomic number of an absorbed substance. Thus, given 34 as the atomic number of Se and 50 as the effective atomic number of CdTe, then sensitivity improved by a factor of approximately 6.9 can be expected. However, when CdTe or CdZnTe is used as the photoconductive film 56 of the radiographic 2-D image detector instead of a-Se, the following problem occurs.
In conventional case of a-Se, a film can be readily formed on the active matrix substrate by means of vacuum deposition at room temperature. In contrast, in case of CdTe or CdZnTe, a film is formed by means of MBE (Molecular Beam Epitaxy) or MOCVD (Metal Organic Chemical Vapor Deposition), and particularly, the latter is considered suitable to form a film over a large-area substrate.
However, when a film of CdTe or CdZnTe is formed by means of MOCVD from raw materials including organic cadmium (DMCd), organic tellurium (DETe or DiPTe), etc., the film has to be formed at a temperature as high as 400xc2x0 C. because the thermal decomposition temperature of DMCd is approximately 300xc2x0 C., and those of DETe and DiPTe are approximately 400xc2x0 C. and 350xc2x0 C., respectively.
The TFT elements formed on the active matrix substrate are generally made out of a film of a-Si or p-Si with addition of hydrogen (H2) at a temperature ranging from 300 to 350xc2x0 C., so that the resulting TFT elements have better semiconductivity. The TFT elements thus formed can withstand temperatures up to 300xc2x0 C., and if the TFT elements are heated above 300xc2x0 C., hydrogen is released from the a-Si film or p-Si film, thereby deteriorating the semiconductivity.
Thus, it is quite difficult to form a film of CdTe or CdZnTe on the active matrix substrate by means of MOCVD because of the high film-forming temperature.
It is therefore an object of the present invention to provide a 2-D image detector with good responsivity and applicable for motion pictures by forming a semiconductor material, such as CdTe and CdZnTe, on the active matrix substrate at a low temperature of 300xc2x0 C. or below.
In order to fulfill the above and other objects, a 2-D image detector of the present invention is characterized by being furnished with;
an active matrix substrate having a pixel alignment layer including switching elements and charge accumulating capacitors having pixel electrodes connected to the switching elements;
a counter substrate having an electrode layer and a photoconductive semiconductor layer, placed in such a manner that the semiconductor layer opposes the pixel alignment layer of the active matrix substrate; and
a conductive connecting layer, provided between the active matrix substrate and counter substrate, for connecting the active matrix substrate to the counter substrate,
at least one of the active matrix substrate and counter substrate being divided into a plurality of pieces.
According to the above arrangement, the active matrix substrate including the pixel alignment layer is electrically and physically connected to the counter substrate including the electrode sections and semiconductor layer. Hence, the active matrix substrate and counter substrate can be manufactured separately, thereby making it unnecessary to form the semiconductor layer directly on the active matrix substrate.
Accordingly, some kinds of materials having good sensitivity to radial rays, such as X-rays, visible light, infrared light, etc. could not have been used in conventional methods due to the withstand temperature of the switching elements, but the above arrangement allows the use of such materials as the semiconductor layer.
In addition, the 2-D image detector is arranged in such a manner that at least one of the active matrix substrate and counter substrate is divided into a plurality of pieces, and each of the smaller divided pieces can be manufactured inexpensively at high yield. Further, by dividing at least one of the substrates, a pressing force applied when connecting the two substrates by means of the conductive connecting layer can be reduced in accordance with the number of the divided pieces. For example, in case that the counter substrate is composed of 12 divided pieces which are laminated sequentially, a total of the pressing forces required to laminate the 12 divided pieces is smaller than the pressing force required to laminate the non-divided counter substrate by one digit or more.
Consequently, even if the areas of the active matrix substrate and counter substrate are enlarged, an increase of the manufacturing costs can be suppressed without lowering the yield. Further, since the pressing force required to connect the two substrates can be reduced, a pressing device can be downsized correspondingly. Moreover, the pressure can be more readily applied uniformly across the substrate composed of the divided pieces.
It should be noted that either the active matrix substrate or counter substrate is divided into a plurality of pieces, or the both substrates are divided into a plurality of pieces, and the cost can be saved in accordance with the number of divided pieces.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.